Coal is a most valuable form of energy. It is estimated that in the United States there is more energy available in coal than in petroleum, natural gas, oil shale, and tar sands combined. Recent energy shortages, together with the availability of abundant coal reserves and the continuing uncertainties in the availability of crude oil, have made it imperative that methods for converting coal into a more useful energy source be developed.
A number of methods are being explored for this purpose. One such method employs gasification techniques, such as destructive distillation, to effect the conversion of coal to a low or medium Btu gas. In another method, high pressure hydrogenation is utilized to liquefy the coal, making it more suitable for transport and burning. In still another more recently developed technique, termed the "chemical surface treatment" method, raw coal is pulverized to a fine mesh size and is chemically treated to treat the surface of the coal with a monomeric compound in the presence of chemical initiators and catalyst. According to the "chemical surface treatment" method the treated coal is then immediately separated from ash and sulfur and cleaned coal is recovered or, alternatively, formed into a liquid mixture, such as a coal-oil mixture. The coal-oil mixture is thereafter treated with additional materials and a stabilizing agent to form a gel or thixotropic mixture, thereby providing a clean or relatively clean stable composition ready for transport, storage, burning or further processing.
Among the many advantages of the chemical surface treatment method are that severe process conditions of, e.g., temperature and pressure, are avoided. In addition, the method achieves high separations of ash and sulfur using cleaning water, without significantly reducing the Btu value of the coal, a problem usually encountered when cleaning fine size coal with cleaning water, that is, the loss of coal in the cleaning water and the retention of water in the product coal. As a further advantage, the process is readily adapted for use at locations near or at the mine site, making it particularly suitable for reducing the transportation difficulties normally associated with coal processing.
In U.S. patent applications Ser. Nos. 114,357 and 114,414, each filed Jan. 22, 1980, and in U.S. Government Report No. 2694 titled "Fuel Extension by Dispersion of Clean Coal in Fuel Oil", all incorporated herein by reference, the above-described chemical surface treatment technique is disclosed for forming coal into a coal-oil mixture. In summary, according to the chemical treatment method, coal is first cleaned of rock and the like and pulverized to a fine size of about 48 to 300 mesh. The pulverized coal, now in the form of a water slurry, is then treated with a monomeric compound, usually in the presence of a liquid organic carrier, a reaction initiator and a reaction catalyst. The chemical treatment of the coal is adapted to make the coal both hydrophobic and oleophilic. Coal particles so treated are readily separated from unwanted ash and sulfur using oil and water separation techniques. The coal, which is now substantially cleaned of ash and sulfur, is then dried to a water content level suitable for further processing or recovery. The dried coal is thereafter formed into a coal-oil mixture, where it is again subjected to a chemical surface treatment using additional additives. The coal-oil mixture is thereafter treated with a gelling agent to form the coal-oil mixture in the form a stable gel or thixotropic mixture.
The coal-oil mixture product thus produced is advantageously non-settling and enjoys a dispersion stability normally difficult to achieve and maintain without frequent stirring, the addition of further additives or an inordinate amount of fine grinding. In addition, the mixture thus formed is thixotropic, allowing for ready pumpability on subjection to shearing or pumping forces.
It will be seen that the chemical surface treatment technique offers considerable advantages in providing coal as a useful energy source. Although the technique is attractive, however, it would be highly desirable that improvements be developed to make the process even more advantageous. For example, it would be highly desirable that the amount of drying which the cleaned coal is subjected to prior to forming the coal-oil mixture could be reduced, without adversely affecting the gel forming process. Such a decrease in drying would significantly improve the overall efficiency of the process, and advantageously would reduce or eliminate the need for burdensome and expensive drying equipment, such as large scale thermal dryers. Thermal drying equipment, e.g., is both unduly expensive and time consuming for large scale coal operations. The need for its use could seriously detract from the attractiveness of the process.
It would also be highly desirable to reduce the amount of monomeric compound utilized to form the coal-oil mixture. A reduction of this reactant would minimize the amounts of processing equipment and treatment time needed and would also reduce the need for undue amounts of raw materials other than coal, making the process even more attractive.
Accordingly, it is an object of the present invention to provide stable coal-oil mixtures and methods for forming coal-oil mixtures.
It is another object of the invention to provide a method for forming coal-oil mixtures in gel or thixotropic form, using coal of high water content.
It is still another object of the invention to provide a method for forming coal-oil mixtures in gel or thixotropic form, using lesser amounts of gelling agents than heretofore thought necessary to form stable mixtures.
Another object of the present invention is to provide a technique for forming stable coal-oil mixtures using coal of high water content, less drying and less reactant materials, which technique would be readily integrated into an overall process for converting raw coal into a stable coal-oil mixture.
These and other objects will become apparent from the following summary of the invention, taken in conjunction with the accompanying detailed description and drawing figures.